1. Field of the Invention
The invention relates to heating mechanisms for process chambers, particularly, heating mechanisms for chemical vapor deposition chambers.
2. Description of Related Art
Chemical vapor deposition (CVD) is a popular process for depositing various types of films on substrates and is used extensively in the manufacture of semiconductor-based integrated circuits such as, for example, the processing of semiconductor wafers to form individual integrated circuit device. In typical CVD processing, a wafer or wafers are placed in a deposition or reaction chamber and reactant gases are introduced into the chamber and are decomposed and reacted at a heated surface to form a thin film on the wafer or wafers.
In general, there are single-wafer and multi-wafer CVD reaction chambers in use today. Multi-wafer reaction chambers typically resemble vertical furnaces capable of holding, for example, 25 wafers or more. For low pressure CVD (LPCVD), for example, 0.25-2.0 torr, for the deposition of Si3N4 or polysilicon, a typical deposition time for a multi-wafer chamber might be several hours. Si3N4, for example, is formed at a temperature between 700-800xc2x0 C. and a deposition time of 4-5 hours depending upon layer thickness in a multi-wafer chamber.
A second type of CVD reaction chamber is a single-wafer chamber in which a wafer is supported in the chamber by a stage or susceptor. The susceptor may rotate during the reaction process. For an LPCVD Si3N4 deposition, for example, a suitable layer thickness may be produced at 700-800xc2x0 C. in about two minutes.
In general, there are two types of heating schemes used in CVD systems: resistive heating schemes that utilize a resistive heating element localized at the wafer, and radiant heating schemes that use a radiant heating element such as a lamp or lamps usually placed outside the reaction chamber. Resistive heating schemes in a single-wafer chamber generally incorporate the resistive heating element directly in the stage or susceptor that supports the wafer in the chamber. In this manner, the reaction produced during the deposition may be generally more localized at the wafer.
In single-wafer resistive heating schemes that utilize a heating element within a stage or susceptor that supports a wafer, the heating element is typically a thin layer of conductive material, such as a thin coiled layer (about 2 mils) of a molybdenum (Mo) material formed in a single plane of the body of the susceptor. This design may be described as a xe2x80x9csingle-zone resistive heater,xe2x80x9d the xe2x80x9czonexe2x80x9d description referring to the location of the heating element in a single plane in the body of the stage or susceptor. The CVD reaction in which the resistive heaters are used typically has a temperature compatibility to approximately 550xc2x0 C. At higher temperatures, temperature uniformity becomes problematic. One reason is that heat loss in a resistive heater increases with higher temperatures, particularly at the edges of the stage or susceptor. Single-zone resistive heaters typically do not have the ability to compensate for differences in heat loss across the stage or susceptor. The pressure in a chamber will also modify the temperature stability of single-zone resistive heaters.
In addition to providing the requisite temperature, the resistive heating element must also be amenable to the chemical environment in the reaction chamber including high temperature and chemical species. One solution to the compatibility consideration in prior art single-zone resistive heaters is to form the susceptor of aluminum nitride (AlN) with the heating element formed inside the susceptor.
Radiant heating schemes generally position lamps behind heat-resistant protective glass or quartz in the reaction chamber. Since the entire chamber is heated by the lamps, the CVD reaction occurs throughout the chamber.
Radiant or lamp heating schemes offer the benefit of generating a high chamber temperature and controlling that temperature better than resistive heating schemes. However, since radiant heating schemes utilize heating elements, e.g., lamps, placed outside of the reaction chamber, the ability to control the temperature in the chamber becomes more difficult as the chamber walls become coated with chemicals or other materials or reaction products used in the reaction chamber. Thus, as the materials used in the chamber deposit on the chamber glass or quartz, for example, the effectiveness of the heating is reduced and the process performance is effected.
In this regard, a reaction chamber used in a radiant heating scheme must be cleaned often. A typical cleaning agent is nitrogen trifluoride (NF3). In Si3N4 CVD processes, for example, Si3N4 reaction products form on the chamber walls and other components inside the chamber, such as a quartz window(s). Si3N4 is difficult to clean from a reaction chamber with a cleaning agent like NF3. The cleaning temperature generally must be high in order to dissociate the NF3 and provide enough thermal energy to clean Si3N4. If the cleaning temperature is high, the NF3 will also attack components in the chamber, such as the susceptor. A remote plasma source used to energize the NF3 can reduce the cleaning temperature but activated NF3 species (particularly radicals) tend to attack quartz components. Therefore, currently there is no effective cleaning solution for radiant-based chambers. Since the walls of the reaction chamber are not easily cleaned with NF3, Si3N4 material accumulates and shortens the lifetime of the chamber.
In LPCVD reactions, temperature uniformity is generally important. The surface reaction associated with a CVD process can generally be modeled by a thermally activated phenomenon that proceeds at a rate, R, given by the equation:
R=oe[xe2x88x92Ea/kT]
where Ro is the frequency factor, Ea is the activation energy in electron volts (eV), and T is the temperature in degrees Kelvin. According to this equation, the surface reaction rate increases with increasing temperature. In a LPCVD process such as a Si3N4 deposition, the activation energy (Ea) is generally very high, on the order of 0.9-1.3 eV. Accordingly, to obtain a uniform thickness across the wafer, the temperature uniformity across the wafer should be tightly controlled, preferably on the order of xc2x12.5xc2x0 C. or less for temperatures around 750xc2x0 C.
Prior art single-wafer radiant heating schemes offer acceptable temperature uniformity even at higher temperatures (e.g., 750xc2x0 C.) when the chamber is clean. However, as materials accumulate on the walls of the chamber, temperature uniformity becomes difficult.
It is also difficult to obtain a uniform high temperature (e.g., 700-750xc2x0 C.) across a wafer with a single-zone resistive heater. As noted, in general, heat loss is not uniform across the surface of a susceptor at higher temperatures. A single-zone heater cannot compensate, for example, for a greater heat loss toward the edges of the susceptor than at its center. Thus, temperature uniformity is a problem.
A second problem with single-zone resistive heaters such as described above and temperatures of 750xc2x0 C. is problems associated with localized heating. At high temperatures, single-zone heaters exhibit concentrated localized heating associated with high density power applied to the heating element at a localized area. Consequently, temperature uniformity is affected. A third problem with single-zone resistive heaters is that variations in manufacturing of the heating element can cause fluctuations in performance of a heating element which can lead to non-uniformity. The single-zone heater cannot be adjusted to compensate for the manufacturing variation. Further, at high temperature operation, single-zone heaters have shorter lifetimes due to the high power density applied at the power terminals and to the heating elements.
Still further, prior art resistive heaters and chambers that provide such heaters offer limited dynamic temperature measurement. In general, the only dynamic temperature measurement (i.e., real-time temperature measurement) is provided by a thermocouple placed generally at the center of the susceptor at a point below the surface of the susceptor. The temperature measurement (such as by a thermocouple) may provide an accurate temperature measurement of the temperature at the center of the susceptor, but cannot provide any information about the temperature at the edges of the susceptor. Thermal cameras that view the temperature within the chamber from a vantage point outside the chamber have been employed but generally only offer static information about the temperature in the chamber. Any changes to the chamber pressure associated with adjusting the CVD process recipe also tend to play a role in the ability to control the reaction temperature in the chamber. Thus, single-zone resistive heating schemes are generally limited to operating at one particular temperature and pressure. Changes to either the chamber temperature or the chamber pressure negatively effect the temperature uniformity. Thus, such single-zone-heating schemes are inadequate for high temperature CVD processes.
What is needed is a reaction chamber and a heating scheme for a reaction chamber compatible with high temperature operation, e.g., on the order of 700xc2x0 C. or greater, that is chemically resistant to the elements and achieves high temperature uniformity localized at a reaction site.
A heating apparatus is disclosed. In one embodiment, the heating apparatus includes a stage or susceptor comprising a surface having an area to support a wafer and a body, a shaft coupled to the stage, and a first and a second heating element. The first heating element is disposed within a first plane of the body of the stage. The second heating element is disposed within a second plane of the body of the stage at a greater distance from the surface of the stage than the first heating elements. According to this embodiment, a multi-zone heating apparatus is disclosed defined by the first and second heating element. In this manner, the invention allows individual control of at least two distinct heating zones of a stage thus increasing the temperature control and temperature uniformity of the stage as compared to prior art single-zone heating apparatuses.
In one aspect, the heating apparatus is a resistive heater capable of operating at high temperatures and providing enhanced temperature uniformity over single-zone resistive heaters. Each heating element may be separately controlled to maintain a collectively uniform temperature across the surface of the stage. For example, in the situation where heat loss is greater at certain areas of the stage, heating zones associated with those areas may be supplied with more resistive heat to maintain a chosen operating temperature despite the heat loss. One way this is accomplished is by varying the resistance of a multiple heating elements across an area of the stage. Where, for example, heat loss through the shaft is determined to be greater than the heat loss at other areas of the stage, the resistance of one heating element in area of the stage corresponding with (e.g., over) the shaft is increased. Similarly, where heat loss at the edge at other area of the stage is determined to be greater than the heat loss at other areas, the resistance of one heating element in an area corresponding with the edge area of the stage is increased.
Also disclosed is a reactor comprising, in one embodiment a chamber and a resistive heater. The resistive heater includes a stage disposed within the chamber including a surface having an area to support a wafer and a body, a shaft coupled to a stage, a first heating element disposed within a first plane of the body of the stage, and a second heating element disposed within a second plane of the body of the stage. In one aspect, the power density of the first heating element is greater than the power density of the second heating element in an area corresponding with a first portion of the stage area. At the same time, the power density of the first heating element is less than the power density of the second heating element in an area corresponding with a second portion of the stage area.
As described, the reactor of the invention provides a multi-zone resistive heater, such as a single-wafer heater, including at least two resistive heating elements disposed within separate planes of a stage or susceptor. The distinct heating elements allow, in one instance, separate areas of the stage to be individually regulated by varying the power density of the individual heating elements in different areas of the stage. In one embodiment, by placing the first heating element at a position closer to the surface of the stage than the second heating element, a greater power density can be supplied to the second heating element to account for greater heat losses at areas associated with the edge of the stage while minimizing potential localized xe2x80x9chot spotsxe2x80x9d associated with the greater power density. Multiple temperature sensors associated with one embodiment of the reactor offer the opportunity to more uniformly control the temperature of the resistive heater than prior art reactors having only a single thermocouple in the center of the susceptor.
A resistive heating system for a chemical vapor deposition apparatus is further disclosed. The heating system includes, in one embodiment, a resistive heater comprising a stage including a surface having an area to support a wafer and a body, a shaft coupled to the stage, a first heating element, and a second heating element disposed within distinct planes of the body of the stage. The heating system of the invention provides a multi-zone resistive heater with at least two distinct heating elements to control the temperature of the heater which improves the temperature uniformity in, for example, a high-temperature CVD process, including process conditions operated at temperature in excess of 700xc2x0 C. (e.g., LPCVD)
A method of controlling the temperature in the reactor is still further disclosed. In one embodiment, the method comprises supplying a power to a first resistive heating element disposed within a first plane of the body of a stage of a resistive heater and a second resistive heating element disposed within a second plane of the body of the stage. The method also comprises varying a resistance of at least one of the first resistive heating element and the second resistive heating elements in at least two areas of the stage.
Additional embodiments of the apparatus, the reactor, the heating system, and the method of the invention, along with other features and benefits of the invention are described in the figures, detailed description, and claims set forth below.